PHOTOSYNTHETIC DECLINE FROM HIGH-TEMPERATURE STRESS DURING MATURATION OF WHEAT .2. INTERACTION WITH SOURCE AND SINK PROCESSES

High temperature stress reduces grain growth in wheat (Triticum aestivum L.) by altering source activity and sink capacity. The impact of stress on source and sink interactions in two wheat cultivars of differing source thermotolerance was monitored by analysis of chlorophyll fluorescence transients, Fv (variable fluorescence) and PSM (peak, stationary, maximum), of attached flag leaves on intact and decapitated tillers grown at optimum (20°C) and stress (35°C) temperatures after anthesis. The thermotolerant cultivar Waverly had reduced Fv and PS quenching and a large increase of SM during heat stress. The less thermotolerant cultivar, Len, exhibited increased Fv and PS quenching and a small increase of SM. Fluorescence induction was similar in intact and decapitated tillers of Len, indicating diminished sinksource interaction during heat stress. The present results and previous observations of photosynthetic activities indicate that cyclic electron transport and photophosphorylation in flag leaves of the thermotolerant cultivar were stimulated by sink demand (increased SM in intact plants). Reduced grain development in the thermolabile cultivar resulted from limited capacity to support cyclic electron transport and photophosphorylation (slight increase in SM of intact plants and large reduction of Cytochrome f/b6-mediated electron transport capacity). It was concluded that heat stress injures the photosynthetic apparatus during reproductive growth of wheat and that diminished source activity and sink capacity may be equally important in reducing productivity.